Many applications in science and industry require an apparatus that creates a controlled amount of a fluid introduced into another fluid. For instance, some material corrosion testing applications require such an apparatus to determine the reaction of a material over time upon exposure to a controlled corrosive vapor-gas mixture. Tests are typically performed using a higher vapor concentration of the vapor-gas mixture, in an effort to decrease the duration of the test. Data are then extrapolated to model the material's reaction over time. However, because the results are extrapolated, any error caused by an imprecisely controlled vapor-gas mixture will be magnified. Thus, an apparatus that creates a controlled testing environment with a gas having a precise flow rate and a precisely controlled vapor-gas mixture is critical to ensure the accuracy of these types of tests.
U.S. Pat. No. 6,182,951—“Method and apparatus for providing a precise amount of gas at a precise humidity,” Hallman, Jr., et al., disclosed a system for controlling the percentage of a fluid mixed with another fluid. However, the system disclosed therein is affected by changes in pressure or flow rate that occur in the process application to which the system is connected. Such variations affect the mixture percentage. What is needed, therefore, is an improved system that is less sensitive to changes in process pressures and flow rates.